1. Field of the Invention
The present invention relates to a method for sensing nuclear magnetic resonance signals, and in particular to such a method which can be used with gradient signals having an arbitrary shape.
2. Description of the Prior Art
Magnetic resonance imaging devices, and methods for operating such devices, are known wherein one or more regions of an examination subject are excited with RF pulses and the resulting nuclear magnetic resonance signals are phase-coded and/or frequency coded dependent on the point of origin by switched magnetic field gradients having a rectangular pulse shape. The nuclear magnetic resonance signals are sensed in the time domain, and the samples acquired in this manner are entered into a measurement matrix in the K-space as measured values. The measurement matrix is then subjected to a Fourier transformation for imaging.
As noted above, in conventional magnetic resonance imaging systems and methods for operating those systems, magnetic field gradients having a rectangular pulse shape are usually employed or, if the gradient field pulse shape was non-rectangular, only that region of the gradient pulses in which the gradient has a constant value were used, for reading out the nuclear magnetic resonance signals. Given fast pulse sequences as are necessary, for example, in the echo planar method, it is extremely difficult to generate rectangular gradient pulses with sufficient amplitude. If the read out interval is limited to the region of the gradient pulse having a constant gradient value, much of the pulse is unusable, thereby wasting read out time.
A magnetic resonance imaging method making use of the aforementioned echo planar sequences is described in European application 0 076 054, wherein sinusoidal gradients are used. To avoid image distortions, sensing of the measured signals does not take place equidistantly in the time domain, but instead takes place equidistantly in the K-space. The chronologically non-equidistant sensing required in this method is difficult to achieve, and cannot be accomplished in standard nuclear magnetic resonance imaging devices.